1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus and a signal reading method for the same. More particularly, the present invention relates to a solid-state image pickup apparatus advantageously applicable to a digital camera, an image input apparatus or the like including an image sensor with high pixel density, e.g., more than several millions of pixels, particularly operable in a preliminary pickup mode.
2. Description of the Background Art
To implement image quality comparable with one available with a silver halide photo-sensitive type of film, there have been proposed various technologies for increasing the number of pixels of a digital camera that electrically shoots a scene. Japanese patent laid-open publication No. 136391/1998, for example, discloses a solid-state image pickup apparatus constructed to optimize the spatial sampling of an image, to shift pixels with respect to each other in such a manner as to enhance efficient receipt of light, and to reduce moire and other aliasing signals.
A digital camera of the type including an image sensor provided with high pixel density is extensively used and directed toward high image quality. It is a common practice with this type of digital camera to effect, before the actual pickup or still shot of a desired scene, AE/AF (Automatic Exposure/Automatic Focusing) operation and movie drive that causes the scene being picked up to appear on an LCD (Liquid Crystal Display). This, however, brings about a problem that the high pixel density increases the period of time necessary for signal charges resulting from the pickup to be read out and thereby lowers the frame rate. It is to be noted that high pixel density refers to more than 1,000,000 pixels or so-called megapixels.
To increase the frame rate, signal charges generated in the image sensor may be read out while being reduced, or thinned, in the vertical direction, as proposed in the past. Specifically, assume that drive frequency CLK for reading out all of 1500,000 pixels (1,280xc3x971,024) by progressive scanning is 12.2725 MHZ. Then, a single horizontal synchronizing period (1H) and a single vertical synchronizing period (1V) are 1,560 CLK and 1,050H, respectively, so that the frame rate is {fraction (1/7.5)} second. When the signal charges are reduced to one-half in the vertical direction, 1H needs the same period of time while 1V is 525H, resulting in a frame rate of 66.7 milliseconds, i.e., {fraction (1/15)} second. Even when the signal charges are reduced to one-fourth in the vertical direction, 1V is 262.6H, and therefore the frame rate is as long as 33.4 millisecond or {fraction (1/30)} second.
Assume that 1,500,000 pixels are read out by progressive scanning and displayed by the movie drive and progressive scanning in the conventional image size, i.e., 640xc3x97480. Then, the pixels are reduced to one-half in the horizontal and vertical directions under the above-described conditions. As a result, the number of pixels in the horizontal direction and the number of pixels (lines) in the vertical direction are as great as 640 and 525, respectively. Reduction to one-fourth in the vertical direction effects reduction to one-half in the horizontal direction and thereby reduces the number of lines in the vertical direction to 262.5, improving the frame rate. However, because the number of lines reduced in the vertical direction is short of 480, interpolation must be executed in the vertical direction in order to match the number of lines to the desired number.
On the other hand, in the horizontal direction, all of the 1,280 pixels are read out and then reduced to 640 pixels at the subsequent signal processing stage. It will therefore be seen that strict consideration is not given to the improvement in frame rate in reducing the pixels in the horizontal direction. This is apt to prevent the operator of the camera from missing an adequate actual pickup timing.
It is therefore an object of the present invention to provide a solid-state image pickup apparatus capable of improving the signal output rate during preliminary pickup despite high pixel density or image quality and preliminarily reading out signals in color without effecting actual pickup to follow, and a signal reading method for the same.
In accordance with the present invention, a solid-state image pickup apparatus includes an image pickup section, a signal processing section for executing digital signal processing, and a signal feeding section. The image pickup section includes photosensitive cells arranged bidimensionally and each being shifted from the adjoining photosensitive cells in the horizontal and vertical directions for photoelectrically transducing incident light. A color filter has color filter segments each being positioned in front of a particular photosensitive cell in the direction of light incidence for separating colors of incident light representative of a scene. Vertical transfer paths transfer signal charges, which are read out of the photosensitive cells, in the vertical direction corresponding to the direction of columns of the bidimensional arrangement of the photosensitive cells. A horizontal transfer path extends in the direction substantially perpendicular to the vertical transfer paths for transferring the signal charges input thereto from the vertical transfer paths in response to horizontal drive signals. Transfer electrodes each are positioned between a particular photosensitive cell and the vertical transfer path adjoining the photosensitive cell. The signal feeding section feeds transfer timing signals to the transfer electrodes for causing the signal charges to be delivered from the photosensitive cells to the vertical transfer paths. Also, the signal feeding section feeds vertical drive signals to the vertical transfer paths for causing the signals charges to be transferred along the vertical transfer paths to the horizontal transfer path. Further, the signal feeding section feeds the horizontal drive signals to the horizontal transfer path for causing the signal charges to be transferred along the horizontal transfer path. The signal feeding section causes the signal charges to be transferred in a particular manner in each of an actual pickup mode, in which the signal charges are read out of substantially all of the photosensitive cells, and a preliminary pickup mode preceding the actual pickup mode. The signal processing section includes an image signal generating circuit assigned to the preliminary pickup mode for performing calculations with the signal charges read out in a mixture to thereby generate a luminance signal and chrominance signals.
Also, in accordance with the present invention, a signal reading method begins with the step of preparing the image pickup section having the above-described configuration. Transfer timing signals are fed for rendering the transfer electrodes conductive to thereby transfer the signal charges from the photosensitive cells to the vertical transfer paths. Vertical drive signals are fed to the vertical transfer paths for transferring the signal charges along the vertical transfer paths to the horizontal transfer path. Further, in a preliminary pickup mode, horizontal drive signals are fed to the horizontal transfer path for mixing some of the signal charges to thereby output mixed signals. The mixed signals are transferred along the horizontal transfer path with a timing for improving a horizontal drive rate being adjusted. As a result, the signal charges are read out of the photosensitive cells as signals in a preliminary pickup mode preceding an actual pickup mode in which signal charges are read out of substantially all of the photosensitive cells.
Further, in accordance with the present invention, a signal reading method begins with preparing the above-described image pickup section. When the color filter has a G (green) square lattice, RB (red and blue) full-checker pattern, transfer timing signals are fed for rendering the transfer electrodes conductive to thereby transfer the signal charges from the photosensitive cells to the vertical transfer paths. Vertical drive signals are fed to the vertical transfer paths for transferring the signal charges along the vertical transfer paths to the horizontal transfer path. Further, in a preliminary pickup mode, horizontal drive signals are to the horizontal transfer path for reading out the signal charges having been mixed during horizontal transfer along the horizontal transfer path while adjusting a timing for mixing some of the signal charges. The resulting mixed signal charges are digitized to thereby output pixel data. A luminance signal and chrominance signals representative of a scene being picked up are generated on the basis of the above pixel data in a preliminary pickup mode preceding an actual pickup mode in which the charges are read out of substantially all of the photosensitive cells.